1. Field of the Invention
The present invention relates to a camera incorporating therein an aperture member which is variable in an aperture caliber including a completely closed condition and changes over to an aperture caliber after an alteration with a predetermined response speed in accordance with an alteration instruction of the aperture caliber, wherein a subject light incident through an aperture of the aperture member is received by an imaging device to create image data.
2. Description of the Related Art
Recently, video cameras capable of photographing still pictures have come into wide use.
In such video cameras, in order to ensure clearness needed for still pictures, the number of pixels of CCD to be loaded is increased.
By the way, some video cameras adopt an aperture having a galvanometer addresses as a galvanic aperture. In the video cameras, there are recorded sounds as well as images, and thus there is a need to reduce driving sounds of an aperture, which would vary in accordance with brightness of field. The reason why some video cameras adopt a galvanic aperture is that the galvanic aperture has a feature that it is high in silence property at the time of the driving.
FIG. 1 is a schematic view of a galvanic aperture.
The galvanic aperture shown in FIG. 1 comprises: an aperture blade 103 consisting of a first aperture blade 103a and a second aperture blade 103b; an aperture driving section 100 consisting of a rotatably movable rotor 100a and a post 100b; and a galvanometer 106 having a Hall-effect device 107 which will be described later.
FIG. 2 is an internal structural view of the galvanometer 106.
The galvanometer 106 comprises a rotary shaft 106a, a magnet 106b mounted on the rotary shaft 106a, a driving coil 106c for controlling an amount of rotation of the rotary shaft 106a, the driving coil 106c being mounted on the magnet 106b; and a damping coil 106d. 
The galvanometer 106 generates lines of magnetic force when a current conducts through the driving coil 106c, so that the generated magnetic force is utilized as a driving force for rotating the magnet 106b. Alteration of the peripheral magnetic field of the damping coil 106d by the rotation of the magnet 106b may generate electromotive force on the damping coil 106d to control a current conducting through the driving coil 106c, so that the rotation of the magnet 106b is set up to a predetermined angle. The Hall-effect device 107 detects the strength of the magnetic field and the magnetic polarity at the position on which the Hall-effect device 107 is mounted to obtain rotary angle information of the magnet 106b. 
The galvanic aperture is associated with an aspect that the aperture is apt to vary, since the aperture blade 103 is of a float structure.
FIG. 3 is a view showing a state that an aperture caliber of the galvanic aperture is changed to an aperture caliber of the target.
FIG. 3 shows a state that the aperture caliber varies from the “open end” side to an aperture caliber of the target, and in addition shows an aspect that it takes some time up to the galvanic aperture becomes steady to a predetermined aperture caliber.
By the way, according to the general digital still camera, in case of photography, first, photometry of the subject brightness by the provisional photography is carries out. And after the photographic conditions such as the aperture caliber and the shutter speed at the time of the regular photography are set up in accordance with the thus obtained subject brightness, the photography is carries out in accordance with the photographic instruction.
The above-mentioned photometry is first carried out in a state that the aperture caliber is in the “open end”. At that time, if it is possible to grasp brightness in all areas on the imaging device such as CCD, photographic conditions are set up in accordance with the brightness. However, in the event that there are areas on the imaging device in which the subject brightness cannot be grasped, in order to grasp the brightness of the area, which cannot be grasped, again photometry is carried out, while the aperture caliber is in a state of the “stop”.
In a video camera having such a galvanic aperture, in consideration of the still picture photography, it is considered that there is a need that in case of a failure of grasp of the subject brightness through the first time of photometry, the second time of photometry is carried out while the aperture caliber is steadied to the “stop”. Further, it is considered that the still picture photography is performed in a state that the aperture caliber is steadied to an aperture caliber different from the “stop”, which is set up in accordance with brightness information obtained through the photometry. In view of the fact that the video camera requires a time up to the aperture caliber becomes steady, whenever the aperture caliber is varied for the photometry and the regular photography, there is a possibility that a photography, which is different from a camera operator's intention, is performed.
In view of the foregoing, there is proposed a technology of contributing to speed-up of the variation of the aperture at the “open” side (cf. for example, Japanese Patent Application Laid Open Gazette TokuKai Hei. 4-223450).
FIG. 4 is a view showing variations of F-values of an aperture from the open end to the close end, as a rotary shaft of the galvanometer rotates.
From FIG. 4, it would be understood that even if the rotary shaft of the galvanometer rotates by the same angle, a degree of the variation of the F-values is different in the aperture caliber between the “open” side and the “stop” side.
According to the above-proposed technology (Japanese Patent Application Laid Open Gazette TokuKai Hei. 4-223450), it is intended to contribute to speed-up of an operating speed of the aperture at the “open” side in such a manner that a lowering of rate of change of an aperture area to change of the rotary shaft of the galvanometer at the “open” side as compared with the “stop” side, as shown in FIG. 4, is compensated by means of providing a circuit for causing rate of change of an aperture area to change of a meter angle from the “stop” side to the “open” side to be constant.
However, according to the above-proposed technology (Japanese Patent Application Laid Open Gazette TokuKai Hei. 4-223450), while the operating speed at the “open” side is increased, there remains the above-mentioned problem since the operating speed at the “stop” side is decreased.
The above-mentioned problem is also applicable to a digital still camera adopting a galvanic aperture for the necessity of silence property of driving sounds, for instance, as well as the video camera having a galvanic aperture.